The theme of the proposal is coordinate regulation of neocortical neuronal laminar class specification and the process of progenitor cell proliferation. A succession of projection laminar (VI - II) neuron classes arises from the pseudostratified epithelium (PVE) within the neocortical ventricular zone (VZ) of the embryonic cerebral wall. A set of homeobox transcription factors, including Emx2, Otx1 and Lhx2 inter alia, acting combinatorially appears to play critical roles in their specification. The transcripts of each of these named transcription factors are expressed as an ascending anterior to posterior (AP) gradient in the PVE. Others, in particular Pax6, follow descending AP gradients. A currently favored model holds that the slopes of the transcript concentration gradients are critical to mechanisms of laminar class specification. We propose here that the slopes of these transcript concentration gradients are regulated by a "morphogenetic gradient" mechanism based in turn upon mechanisms regulatory to the PVE proliferative cell cycle. The central hypotheses for the proposal are (1) that a gradient of Q (the fraction of cells leaving the cycle following mitosis) with cell cycle is the "morphogenetic gradient" regulatory to the concentration slopes of the transcripts of Emx2, Otx1 and Lhx2, (2) that the gradient in Q is driven, at least in part, by the cell cycle inhibitor p27Kip1, and (3) that Emx2 (and possibly Otx1 and Lhx2) is also regulatory to Q. There are 3 Specific Aims. Specific Aim 1 will determine if gradients of expression of Emx2, Otx1 and Lhx2 are correlated closely with the length of the G1-phase of the cell cycle (TG1) and Q in wild type animals and if the gradients of these transcription factors vary with Q and not TG1 following p27K1p1 overexpression in a transgenic mouse model where Q and TG1 are dissociated. Specific Aim 2 will determine if down regulation of proliferation in Emx2 null mice is associated with upregulation of Q and with upregulation of p27Kip1, and if in these null mice there is disruption of covariation of Lhx2 and Otx1 with Q. Specific Aim 3 will define a larger set of signal transduction, cell cycle and differentiation related transcripts that regulate with Q and not TG1 in response to p27Kip1 overexpression in the transgenic mice.